The present invention relates to facsimile communications over wireless links, and, more particularly, to facsimile (fax) communications over a cellular communications channel.
A cellular modem pool comprises a number of pairs of modems, in which the data terminal equipment (DTE) ports of each modem pair are cross-connected in a "back-to-back" fashion. This allows the two modems of each pair to interchange data via their DTE ports and thereby isolate that portion of the data connection over the cellular communications channel from that portion of the data connection through the public-switched-telephone-network (PSTN). This isolation allows the use of a data protocol that is better suited to combating the effects of the cellular environment over the cellular portion of the data connection. One example of a cellular-oriented protocol is the "Enhanced Throughput Cellular" (ETC) protocol, developed by AT&T Paradyne. Similarly, on the PSTN side of the data connection, the cellular modem pool concept allows the modems at each endpoint of the PSTN connection to use a more traditional, e.g., V.32bis, land-line oriented protocol. In offering such a service, a cellular provider typically discounts calls through the cellular modem pool so that a data call is cheaper than a cellular voice call.
Unfortunately, routing a data call through a cellular modem pool presents a number of problems. One such problem is the ability to provide call progress information to a calling party. The co-pending U.S. patent application of Exner et al., entitled "Transparent Call Progress," Ser. No. 08/380,872, filed on Jan. 30, 1995, describes a method and apparatus that solves this problem.
However, another problem with a cellular modem pool is facsimile (fax) communications. Fax communications has, in a sense, its own world of protocols and modulation techniques that make it difficult to use a cellular modem pool in a straight forward way to establish a fax call.